This invention relates to a prosthetic arm and more particularly to a body-powered prosthetic arm having a bendable elbow and a terminal gripping device, each of which may be operated independently of the other, but with the same actuation mechanism.
Prosthetic arms generally fall into two categories, these being body-powered arms and motor-actuated arms. The body-powered type prosthetic arm is more widely used because it is less expensive, less complicated in design, does not require batteries, is lightweight, and is more reliable. The motor-actuated type prosthetic arm, however, does provide ease of use and some versatility in the operation of the gripping terminal device as to opening and closing of the gripping element and the pressure applied by the gripping element in grasping an object.
One of the most common body-powered prosthetic arm utilizes what is called a dual cable harnessing system, such as a figure eight harness, for attachment to a person's body and for actuating both the bending of the elbow portion of the arm and the opening and closing of the gripping element, typically a hook. One of the cables in this type system is for enabling one of the two possible movements to take place, i.e., either the bending of the elbow or opening or closing of the hook. The other cable is for causing the bending of the elbow or the opening or closing of the hook, whichever is enabled.
One of the problems with the above dual cable system is that with certain positions of the prosthetic arm, the two possible movements cannot be "decoupled", i.e., the bending of the elbow cannot be "decoupled" from the opening of the hook. Also, in certain other positions, there is insufficient length of actuation cable to effect an opening of the hook when desired. Such is the case, for example, when the hook is raised a certain distance toward the mouth and there is simply no more cable to enable opening the hook to release the object being held.